U.S. patent number 10,043,923 [Application Number 14/691,124] was granted by the patent office on 2018-08-07 for laser doping of crystalline semiconductors using a dopant-containing amorphous silicon stack for dopant source and passivation.
This patent grant is currently assigned to GLOBALFOUNDRIES INC.. The grantee listed for this patent is GLOBALFOUNDRIES INC.. Invention is credited to Deborah A. Neumayer, Katherine L. Saenger.
United States Patent |
10,043,923 |
Neumayer , et al. |
August 7, 2018 |
Laser doping of crystalline semiconductors using a
dopant-containing amorphous silicon stack for dopant source and
passivation
Abstract
Techniques and structures for laser doping of crystalline
semiconductors using a dopant-containing amorphous silicon stack
for dopant source and passivation are provided. An example method
includes forming a dopant-containing amorphous silicon layer stack
on at least one portion of a surface of a crystalline semiconductor
layer; and irradiating a selected area of the dopant-containing
amorphous silicon layer stack, wherein the selected area of the
dopant-containing amorphous silicon layer stack interacts with an
upper portion of the underlying crystalline semiconductor layer to
form a doped, conductive crystalline region, and each non-selected
area of the dopant-containing amorphous silicon layer stack remains
intact on the at least one portion of the surface of the
crystalline semiconductor layer.
Inventors: |
Neumayer; Deborah A. (Danbury,
CT), Saenger; Katherine L. (Ossining, NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
GLOBALFOUNDRIES INC. |
Grand Cayman |
N/A |
KY |
|
|
Assignee: |
GLOBALFOUNDRIES INC. (Grand
Cayman, KY)
|
Family
ID: |
50431779 |
Appl.
No.: |
14/691,124 |
Filed: |
April 20, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150228487 A1 |
Aug 13, 2015 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
13646120 |
Oct 5, 2012 |
9112068 |
|
|
|
13645926 |
Oct 5, 2012 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
31/068 (20130101); H01L 31/022425 (20130101); H01L
21/268 (20130101); H01L 21/02592 (20130101); H01L
31/1804 (20130101); H01L 21/2257 (20130101); H01L
31/0747 (20130101); H01L 21/02686 (20130101); H01L
21/02532 (20130101); H01L 21/76838 (20130101); Y02P
70/50 (20151101); Y02E 10/547 (20130101) |
Current International
Class: |
H01L
31/00 (20060101); H01L 31/0747 (20120101); H01L
31/18 (20060101); H01L 31/068 (20120101); H01L
31/0224 (20060101); H01L 21/02 (20060101); H01L
21/768 (20060101); H01L 21/268 (20060101); H01L
21/225 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
59056775 |
|
Apr 1984 |
|
JP |
|
5326989 |
|
Dec 1993 |
|
JP |
|
Primary Examiner: Gardner; Shannon M
Attorney, Agent or Firm: Scully Scott Murphy and Presser
Digiglio; Frank
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser.
No. 13/646,120, filed Oct. 5, 2012, which is a continuation of U.S.
patent application Ser. No. 13/645,926, filed Oct. 5, 2012, both of
which are incorporated by reference herein.
Claims
What is claimed is:
1. A method for forming at least one doped, conductive crystalline
region on a surface of a crystalline semiconductor layer, the
method comprising: forming a dopant-containing amorphous silicon
layer stack on at least one portion of a surface of a crystalline
semiconductor layer, wherein forming the dopant-containing
amorphous silicon layer stack comprises forming a
i-aSiH(bottom)/doped-aSiH(top) layer on the surface of the
crystalline semiconductor layer, and the dopant-containing
amorphous silicon layer stack comprises a purely amorphous phase as
well as an amorphous phase with embedded nanocrystalline and/or
microcrystalline regions; and irradiating a selected area of the
dopant-containing amorphous silicon layer stack, wherein the
selected area of the dopant-containing amorphous silicon layer
stack interacts with an upper portion of the underlying crystalline
semiconductor layer to form a doped, conductive crystalline region,
and each non-selected area of the dopant-containing amorphous
silicon layer stack remains intact on the at least one portion of
the surface of the crystalline semiconductor layer.
2. The method of claim 1, wherein said irradiating comprises
providing pulsed laser radiation to locally heat the selected area
of the layer stack and an upper portion of the semiconductor layer
underlying the selected area of the layer stack.
3. The method of claim 1, wherein said forming a dopant-containing
amorphous silicon layer stack further comprises: forming an
overlayer stack on the dopant-containing amorphous silicon layer
stack, wherein the overlayer stack comprises at least one of a
single or multilayer dielectric coating, a back reflector, a
diffusion barrier, a transparent conductor, and a metallic
conductor, and wherein said irradiating comprises irradiating at
least one selected area of the dopant-containing amorphous silicon
layer stack to form at least one localized crystalline region of
high doping concentration as well as a self-aligned opening in the
overlayer stack above said localized crystalline region.
4. The method of claim 3, comprising: forming a conductive contact
layer over the overlayer stack and exposed localized crystalline
regions remaining after irradiation.
5. The method of claim 1, comprising: forming a blanket transparent
conductor layer over the dopant-containing amorphous silicon layer
stack and localized crystalline regions remaining after
irradiation.
6. The method of claim 5, comprising: forming a metallic conductor
layer on the blanket transparent conductor layer, wherein the
metallic conductor has a grid pattern or a blanket pattern.
7. The method of claim 1, wherein the dopant-containing amorphous
silicon layer stack comprises (i) a lower layer of undoped
amorphous silicon providing a passivating function and (ii) an
upper layer of doped amorphous silicon providing a dopant source
function.
8. The method of claim 7, wherein the lower layer of undoped
amorphous silicon and/or the upper layer of doped amorphous silicon
comprise one of a-SiH, a-Si(Ge)H, a-Ge(Si)H, and a-GeH.
9. The method of claim 8, wherein H content varies from
approximately 5% to approximately 50% atomic percent.
10. The method of claim 7, wherein the lower layer of undoped
amorphous silicon and/or the upper layer of doped amorphous silicon
further comprise carbon (C) and/or a dopant selected from a group
including boron (B), phosphorous (P), arsenic (As), antimony (Sb),
nitrogen (N), gallium (Ga), indium (In), and aluminum (Al).
Description
FIELD OF THE INVENTION
Embodiments of the invention generally relate to electronic devices
and, more particularly, to doping semiconductor solar cell
devices.
BACKGROUND OF THE INVENTION
Challenges exist in obtaining solar cell back surface fields (bsfs)
on a low-to-moderate thermal budget (for example, <400-800 C) in
p-type silicon (Si). Aluminum- (Al-)based bsfs (fabricated by
>800 C alloying of an Al-paste or metallic Al layer) can have
the desired several-micron thickness, but can be difficult to form
at temperatures below 800 C due to the low solid solubility of Al
in Si. While boron (B) dopants have much higher solid solubilities
in Si, the long periods at high temperatures (for example, 900-1050
C) required for sufficient B diffusion can degrade the bulk
lifetime of the Si substrate and/or lead to dopant clustering in
ways that can produce misfit dislocations.
Some back surface field functionality can be provided in
heterojunction with intrinsic thin layer (HIT) cells with the use
of intrinsic amorphous silicon (i-aSiH)/doped-aSiH stacks on Si
substrates (for example, p-Si(substrate)/i-aSiH/p-aSiH and
n-Si(substrate)/i-aSiH/n-aSiH)), but these cells can be difficult
to fabricate due to the narrow process window for providing
i-aSiH/doped-aSiH stacks with aSiH layers thick enough to provide
good passivation yet thin enough to provide sufficient tunneling
current to the back surface metallurgy.
Accordingly, a need exists for a low-temperature, easy-to-integrate
technique for forming B-doped back surface fields in p-type Si.
SUMMARY OF THE INVENTION
In one aspect of the invention, techniques for laser doping of
crystalline semiconductors using a dopant-containing amorphous
silicon stack for dopant source and passivation are provided. An
exemplary method for forming at least one doped, conductive
crystalline region on a surface of a crystalline semiconductor
layer can include steps of forming a dopant-containing amorphous
silicon layer stack on at least one portion of a surface of a
crystalline semiconductor layer, and irradiating a selected area of
the dopant-containing amorphous silicon layer stack, wherein the
selected area of the dopant-containing amorphous silicon layer
stack interacts with an upper portion of the underlying crystalline
semiconductor layer to form a doped, conductive crystalline region,
and each non-selected area of the dopant-containing amorphous
silicon layer stack remains intact on the at least one portion of
the surface of the crystalline semiconductor layer.
In another aspect of the present invention, techniques for forming
a doped, conductive crystalline region on a surface of a
crystalline semiconductor layer can include steps of forming a
dopant-containing amorphous silicon layer stack on at least one
portion of a surface of a crystalline semiconductor layer, and
irradiating the dopant containing amorphous silicon layer stack,
wherein the dopant-containing amorphous silicon layer stack
interacts with an upper portion of the underlying crystalline
semiconductor layer to form a blanket doped, conductive crystalline
region.
These and other objects, features and advantages of the present
invention, particularly those relating to improved solar cell
structures and fabrication methods, will become apparent from the
following detailed description of illustrative embodiments thereof,
which is to be read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A through FIG. 1C are cross-section view diagrams
illustrating an example process schematic for using a
dopant-containing amorphous silicon layer stack to form localized
doped crystalline regions, according to an embodiment of the
invention;
FIG. 2A through FIG. 2C are cross-section view diagrams
illustrating an example process schematic for using an
overlayer-coated dopant-containing amorphous silicon layer stack to
form localized doped crystalline regions, according to an
embodiment of the invention;
FIG. 3A and FIG. 3B are cross-section view diagrams illustrating a
configuration of overlayers that may be used in combination with a
dopant-containing amorphous silicon layer stack to form localized
doped crystalline regions, according to an embodiment of the
invention;
FIG. 4A and FIG. 4B are cross-section view diagrams illustrating an
additional configuration of overlayers that may be used in
combination with a dopant-containing amorphous silicon layer stack
to form localized doped crystalline regions, according to an
embodiment of the invention;
FIG. 5A through FIG. 5G are cross-section view diagrams
illustrating examples of different solar cell structures in which
localized crystalline doped regions may be incorporated as
localized bsfs, according to an embodiment of the invention;
FIG. 6A and FIG. 6B are cross-section view diagrams illustrating a
first example process schematic for using a dopant-containing
amorphous silicon layer stack to form a blanket doped crystalline
region, according to an embodiment of the invention;
FIG. 7A through FIG. 7E are cross-section view diagrams
illustrating a second example process schematic for using a
dopant-containing amorphous silicon layer stack to form a blanket
doped crystalline region, according to an embodiment of the
invention;
FIG. 8A through FIG. 8C are cross-section view diagrams
illustrating examples of different solar cell structures
incorporating blanket bsfs formed according to an embodiment of the
invention;
FIG. 9A through FIG. 9I are cross-section view diagrams
illustrating an example process schematic for forming a solar cell
incorporating an example embodiment of the invention;
FIG. 10A through FIG. 10G are a mix of cross-section and plan-view
diagrams illustrating an example process schematic for forming a
HIT cell incorporating an example embodiment of the invention;
FIG. 11 is a flow diagram illustrating techniques for forming at
least one doped, conductive crystalline region on a surface of a
crystalline semiconductor layer, according to an embodiment of the
present invention; and
FIG. 12 is a flow diagram illustrating techniques for forming a
doped, conductive crystalline region on a surface of a crystalline
semiconductor layer, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS
As described herein, an aspect of the present invention includes
laser doping of crystalline semiconductors. At least one embodiment
of the invention includes creating heavily-doped, conductive
crystalline regions in selected areas of a surface of a crystalline
base semiconductor layer by locally laser-melting a dopant-source
overlayer stack containing doped amorphous silicon (for example,
doped-aSiH in an i-aSiH(bottom)/doped-aSiH(top) bi-layer).
As further detailed herein, the constituents of an
i-aSiH/doped-aSiH stack in laser-irradiated regions (dopants plus
matrix in which the dopants are contained) can largely remain in
the structure after being converted to crystalline form.
Additionally, the laser-melted regions can be blanket or patterned
(dots, lines, etc.). For the case of patterned features, the doped
amorphous silicon stack remains in the structure (between the laser
irradiated regions) as a passivant. Also, in at least one
embodiment of the invention, the laser melting can simultaneously
pattern (form openings in) various other layers (for example,
dielectric layers) over the dopant-containing layers.
Further, the techniques detailed herein can be used and/or
implemented in connection with any structure (particularly silicon
solar cell structures) in which heavily doped crystalline layers
are needed. Additionally, such techniques can be implemented in
cases where there exists a need or desire to avoid high temperature
(for example, greater than 250-400 C) processing.
As also described herein, at least one embodiment of the invention
includes a structure (for example, a solar cell structure)
containing a crystalline semiconductor having a surface in which
heavily doped crystalline regions are disposed in a surrounding
surface region of a relatively lightly-doped semiconductor, wherein
the lightly-doped semiconductor in the field region is passivated
with an amorphous silicon layer stack containing the same dopant as
present in the heavily doped region.
As noted below, at least one embodiment of the invention includes
laser doping using passivating i-aSiH(bottom)/B-doped p-aSiH(top)
bi-layer stacks as dopant sources, wherein the i-aSiH layer
passivates the Si surface and the B-doped p-aSiH layer provides a
source of B dopant to the irradiated area.
FIG. 1A through FIG. 1C are cross-section view diagrams
illustrating an example process schematic for using a
dopant-containing amorphous silicon layer stack to form localized
laser doped crystalline regions. FIG. 1A depicts dopant-containing
amorphous silicon layer stack 100 which includes an amorphous
i-aSiH layer 102 and a B-doped p-aSiH layer 104 disposed on an
underlying Si substrate 106 that is irradiated by patterned
radiation 108. Patterned radiation 108 is typically provided by one
or more pulses of a pulsed laser (pulse length <100 nanoseconds
(ns)) so as to reduce thermal diffusion and bulk substrate heating.
FIG. 1B shows locally melted region 110 (which includes the
irradiated portion of stack 100 as well as an upper portion of
substrate 106 below it) produced by patterned radiation 108. While
region 110 is melted, B dopant diffuses from the source layer 102
portion of the melt into the substrate 106 portion of the melt.
FIG. 1C shows the melted/diffused irradiated region 110 after
crystallization into laser-doped crystalline region 112, where the
crystallization has been templated by the underlying crystalline
substrate 106. Also, the i-aSiH/B-doped p-aSiH (102/104) layer
stack remains as a passivant in the non-irradiated regions.
The laser doping process illustrated in FIG. 1A through FIG. 1C can
alternatively be implemented with one or more overlayers disposed
on the dopant source layer stack. Such overlayers might include
antireflection coatings (ARCs), as well as other dielectric and/or
transparent reflector or barrier layers (such as SiO.sub.2 and/or
SiN).
FIG. 2A through FIG. 2C are cross-section view diagrams
illustrating an example process schematic with a dielectric
overlayer coating or layer stack 230, according to an embodiment of
the invention. In this case, the patterned laser irradiation 108
that induces the i-aSiH/B-doped p-aSiH (102/104) melting also opens
the overlayer to form openings 232 in patterned overlayer 230', as
illustrated in FIG. 2B. FIG. 2C shows melted/diffused irradiated
region 110 after crystallization into laser-doped crystalline
region 112, where the crystallization has been templated by the
underlying crystalline substrate 106. As before, the i-aSiH/B-doped
p-aSiH (102/104) layer remains as a passivant in the non-irradiated
regions. It should be noted that the same overlayer stack 230 might
function both as an ARC during laser processing (where it can
produce an increase in the absorption of the laser) and a
reflection-enhancing layer when disposed in a completed solar cell,
for example, between a Si substrate back surface and Al back
contact layer.
FIG. 3A and FIG. 3B as well as FIG. 4A and FIG. 4B are
cross-section view diagrams illustrating two additional
configurations of overlayers that may be used in combination with a
dopant-containing amorphous silicon layer stack to form localized
laser-doped crystalline regions, according to an embodiment of the
invention. In a first additional configuration, a metallic
overlayer 300 (for example, Al) is disposed on a structure (such as
depicted in FIG. 1A), as shown in FIG. 3A. Patterned irradiation
108 then forms a melted region (such as described in connection
with FIG. 1 and/or FIG. 2) which crystallizes into localized doped
crystalline region 312, as shown in FIG. 3B, where the
crystallization has been templated by the underlying crystalline
substrate 106. Metallic overlayer 300 is typically removed in the
irradiated area, but reflowed at edges of the irradiated area to
make reflowed edge contacts 320 with the laser doped crystalline
region 312.
In a second additional configuration, a metallic overlayer 300 (for
example, Al) is disposed on the structure (such as depicted in FIG.
2A), as shown in FIG. 4A. Patterned irradiation 108 then forms a
melted region (as detailed above), which crystallizes into
localized doped crystalline region 312, as shown in FIG. 4B, where
the crystallization has again been templated by the underlying
crystalline substrate 106. Also, the patterned laser irradiation
108 that induces the i-aSiH/B-doped p-aSiH (102/104) melting
opens/partitions overlayer 230 to form openings 232 and patterned
overlayer 230'. As noted in FIG. 3B above, metallic overlayer 300
is typically removed in the irradiated area, but reflowed at edges
of the irradiated area to make reflowed edge contacts 320 with the
laser doped crystalline region 312.
The presence of metallic overlayers in the stack before laser
processing can reduce peripheral heating/collateral damage to the
dopant-containing amorphous silicon stack 102/104 at the edges of
the irradiated area for cases in which the patterned laser
radiation has a spatially non-uniform intensity or fluence profile
that is high at the center and low at the edges. This can occur
because the Al is only opened in the high fluence center portion of
the irradiated region, resulting in an aperture or mask for
transmission into the substrate that is smaller than the laser spot
dimensions. Low fluence radiation outside of the center region can
be efficiently reflected by the remaining Al, thus reducing
heat-induced depassivation effects at the spot edges. While
incorporating Al into the stack may include an extra step for
certain applications (and thus a cost adder), it would not
necessarily include an extra step in process flows in which an Al
deposition step after laser processing can be replaced by one
before laser processing.
It should be appreciated by one skilled in the art that while FIGS.
1 through 5 depict the case of i-aSiH/doped-aSiH stack containing
B, implementation of the invention with other passivating amorphous
stacks is also contemplated. For example, the i-aSiH/doped-aSiH
stack may include alternative dopants (of either the same or
opposite doping type; for example, phosphorous (P) being an example
of an opposite doping type), as well as carbon (C) and/or germanium
(Ge) replacing some or all of the Si in the aSiH and doped-aSiH
layers, as well as additional doped and undoped aSiH layers. In
addition, it should also be noted that the materials of the
amorphous i-aSiH/doped-aSiH stack can include embedded
nanocrystalline or microcrystalline semiconductor regions, that is,
the term "amorphous" should be taken to include the purely
amorphous phase as well as the amorphous phase with embedded
nanocrystalline or microcrystalline regions. Likewise, while FIGS.
1 through 5 depict the case of a crystalline silicon substrate, the
substrate may include or comprise other semiconductors (and/or
layers of other semiconductors) such as Ge and/or SiGe alloys.
FIG. 5A through FIG. 5G include cross-section view diagrams
illustrating examples of different solar cell structures in which
the localized laser doped crystalline regions may be incorporated
as localized bsfs, in accordance with at least one embodiment of
the invention. The solar cells of FIG. 5A through FIG. 5G are
formed with p-type Si substrates 406, and have a generic front
structure 410 that includes selective emitter with lightly doped
n-type regions 422 and heavily doped n-type regions 424. The
depicted solar cells additionally include ARC 426 and a front
conductive finger/bus grid 428. As would be appreciated by one
skilled in the art, alternative front structures may also be
employed.
Further, in FIG. 5A through FIG. 5G, back structures 450 include
heavily doped p-type crystalline localized laser bsf regions 412
(typically in a pattern of separated dots, but alternatively a
pattern of grid lines), dopant-containing amorphous silicon layer
stack 460 that also provides a passivation function, and a metallic
back contact layer (for example, Al). In the structure of FIG. 5A,
the metallic back contact layer 464 is blanket and directly on both
the local bsf 412 and amorphous silicon stack 460.
The structure of FIG. 5B is similar to the structure of FIG. 5A,
but includes a thin barrier layer 470 (for example, 15 nanometers
(nm) of SiN or Al.sub.2O.sub.3 deposited by plasma-enhanced
chemical vapor deposition (PECVD)) patterned with contact openings
under blanket metallic back contact layer 464 to prevent contact
layer/amorphous stack reaction.
As would be appreciated by one skilled in the art, internal optical
reflectivity at the Si side of a Si/Al interface can be
significantly increased by inserting a transparent layer of a
suitably selected refractive index and thickness between the Si and
the Al; a high internal reflectivity is desirable for high
efficiency cells because photons not absorbed during a first pass
through the cell substrate have a second chance to be absorbed when
reflected back into the cell. The structure of FIG. 5C is similar
to that of FIG. 5B, but with the substitution of patterned
(typically dielectric) back layer stack 472 having a thickness
suitable for functioning as a reflector layer as well as a barrier
(for example, SiN (15 nm)/SiO.sub.2 (90 nm) or SiO.sub.2 (110 nm)
alone). The structure of FIG. 5D utilizes a blanket transparent
conductive reflector layer 474 (for example, 80 nm of Al-doped ZnO
or SnO.sub.2-containing In.sub.2O.sub.3) under blanket metallic
back contact layer 464. This structure has the advantage of
providing a barrier/reflector function without the need for contact
open patterning. The structure of FIG. 5E is similar to that of
FIG. 5D except blanket metallic conductor layer 464 is replaced by
grid-patterned metallic contact layer 476. The structure of FIG. 5F
is similar to the structure of FIG. 5C, except that blanket
metallic contact layer 464 is replaced by patterned metallic
contact layer 478 with opening 480 above localized bsf 412 and
reflowed edge contacts 482. The structure of FIG. 5G is identical
to that of FIG. 5F, except that it further includes both back
contact layer 478 and back contact layer 464.
The processes of FIGS. 1-4 may also be used to form n-type
localized bsfs in n-type Si substrates if p-type dopant-containing
source layer stack 102/104 is replaced with an n-type
dopant-containing source layer stack. Likewise, solar cell
structures analogous to those shown in FIG. 5A through FIG. 5G may
alternatively be fabricated with n-type Si substrates. In such an
instance, p-type dopant-containing source layer stack 102/104 would
be replaced with an n-type dopant-containing source layer stack,
p-type localized laser bsf 412 would be replaced with an n-type
localized laser bsf, and n-type emitter 440/442 would be replaced
with a p-type emitter.
While FIGS. 1 through 5 show patterned (or localized) embodiments
of the invention, it is noted herein that aspects of the invention
may be implemented in large-area (for example, blanket) modes as
well. For instance, a blanket laser BSF embodiment of the invention
can be implemented on a Si substrate by irradiating blanket regions
of a dopant-containing amorphous Si layer stack disposed on a Si
substrate having the same dopant type. The irradiation can be
performed with large-area (for example, 1.times.1 square
centimeters (cm.sup.2)) laser spots (as shown in FIG. 6) or in a
rastered mode in which a small-area spot (for example, 50 .mu.m
diameter, though line shaped spots are also possible) is scanned so
as to completely cover the desired area (as shown in FIG. 7).
Specifically, FIG. 6A depicts, in cross-section view a starting
structure for implementation of a blanket laser bsf on p-type
substrate 106 using a p-doped aSiH stack (104/102). The structure
of FIG. 6A is irradiated with large-area irradiation 508, resulting
in doped conductive crystalline region 512, as shown in FIG. 6B.
Large-area irradiation 508 is typically provided by one or more
pulses of a pulsed laser (pulse length <100 ns) so as to reduce
thermal diffusion and bulk substrate heating. Substrates larger
than the lateral dimensions of irradiation 508 may be irradiated,
for example, in a step and repeat mode. In a typical example of
step and repeat irradiation, the step size would be comparable to
the lateral dimensions of irradiated region.
Accordingly, FIG. 7A through FIG. 7E include cross-section view
diagrams illustrating an example process schematic for fabricating
blanket laser back surface field in a rastered mode. Specifically,
FIG. 7A depicts a starting structure for implementation of a
blanket laser bsf on p-type substrate 106 using a p-aSiH stack
(102/104). The structure of FIG. 7A is irradiated with spot
irradiation 608, resulting in doped conductive crystalline region
612, as shown in FIG. 7B. FIG. 7B through FIG. 7D show successive
stages of the process as spot irradiation 608 is scanned or
rastered to enlarge the size of region 612 to its final size shown
in FIG. 7E. Spot irradiation 608 is typically provided by a pulsed
laser (pulse length <100 ns) so as to reduce thermal diffusion
and bulk substrate heating. It is noted that the irradiated regions
provided by sequential pulses of spot irradiation 608 would
typically have substantial overlap, and that such rastered
irradiation may be viewed as a form of step and repeat irradiation
implemented with a step size significantly smaller than the spot
size.
FIG. 8A through FIG. 8C include cross-section view diagrams
illustrating examples of three different solar cell structures in
which the large-area and/or blanket crystalline doped regions of
the invention may be incorporated as blanket bsfs. Similar to the
solar cells depicted in FIG. 5A through FIG. 5G, the solar cells of
FIG. 8A through FIG. 8C are formed with p-type Si substrates 406,
and have a generic front structure that includes a selective
emitter with lightly doped n-type regions 422 and heavily doped
n-type regions 424. The solar cells of FIG. 8A through FIG. 8C also
include front ARC 426 and a finger/bus grid 428.
The structure of FIG. 8A has a blanket bsf 712 with a blanket
metallic back contact layer 720 (for example, Al). The structure of
FIG. 8B further includes a blanket reflector layer in the form of a
transparent conductor layer 736 between the blanket bsf 712 and
metallic back contact layer 720. The structure of FIG. 8C is
similar to the structure of FIG. 8B, except for the substitution of
a via-patterned (or line-patterned) transparent reflector layer 738
for blanket reflector layer 736. Patterning of transparent
reflector layer 738 may be accomplished, for example, by pulsed
laser processing similar to that shown in FIG. 2, or by
conventional lithography and wet etching. Reflector layer 738,
which typically includes one or more transparent dielectric layers,
and reflector layer 736 have several functions. In addition to the
reflection-enhancing function mentioned in connection with the FIG.
4 layers 472 and 474, reflector layers 736 and 738 can also reduce
the back surface minority carrier recombination velocity because
recombination at bsf/metal interface 712/720 is likely to be worse
than at bsf/reflector interfaces 712/736 and 712/738.
Solar cell structures analogous to those shown in FIG. 8A through
FIG. 8C may alternatively be fabricated with n-type Si substrates
instead of the p-type Si substrates shown. In such instances,
p-type blanket laser bsf 712 would be replaced with an n-type
blanket laser bsf, and n-type emitter 422/424 would be replaced
with a p-type emitter.
FIG. 9A through FIG. 9I include cross-section view diagrams
illustrating an example process schematic for a process flow
incorporating an example embodiment of the invention. Specifically,
FIG. 9A through FIG. 9I depict an example process flow and
structure for a cell that has a generic laser-doped selective
emitter on the cell front and a localized laser bsf (produced via
at least one embodiment of the invention) on the cell back. It
should be noted and appreciated that the order of certain steps is
not critical, and that other steps, shown or not shown, may be
added, combined, substituted with other steps producing comparable
function, or eliminated, depending on the final structure desired,
as would be known to those skilled in the art.
FIG. 9A through FIG. 9F depicts an initial portion of an example
process flow, namely the steps for forming a solar cell front. FIG.
9A depicts selecting a p-type c-Si substrate 106. FIG. 9B depicts
performing a POCl.sub.3 diffusion to form a lightly-doped n-type
emitter layers 422 and 422' on both sides of substrate 106. FIG. 9C
depicts forming passivating ARC layer stack 426 (a layer of PECVD
SiN, for example). FIG. 9D depicts removing the back surface
emitter layer 422' (by a process such as etching in a solution of
tetramethylammonium hydroxide (TMAH)). FIG. 9E depicts forming
heavily doped selective emitter 424 and finger/grid openings 427 in
passivating ARC layer stack 426, which may be accomplished by
forming a n-type (for example, phosphorus) doping source layer (for
example, a phosphosilicate glass (PSG), not shown) on ARC layer
stack 426, irradiating the structure in selected regions with a
patterned pulsed laser radiation to both locally diffuse the dopant
and form finger/grid openings in the ARC layer stack, and then
removing unreacted portions of the doping source layer. Further,
FIG. 9F shows the structure of FIG. 9E after deposition of
conductive front grid/bus structure 428.
FIG. 9G through FIG. 9I show the steps of the example process flow
particular to an embodiment of the invention, namely the steps that
may be used to form a solar cell back that includes laser doped
crystalline local bsf regions. FIG. 9G depicts forming a back
surface p-type dopant-containing aSiH/reflector-barrier stack
comprising i-aSiH layer 102, B-doped p-aSiH layer 104, and
overlayer stack 830. Overlayer stack 830 is typically a transparent
reflector/barrier layer 830 and may include, for example, a lower
layer of PECVD SiN (15 nm) in contact with doped layer 104 and an
upper layer of PECVD SiO.sub.2 (100 nm).
Patterned laser irradiation is applied to the back of structure of
FIG. 9G to form patterned overlayer stack 830' with openings 832
over heavily p-type doped localized bsf regions 850. The
irradiation can, for example, be in a pattern of spaced-apart dots
(for example, 50 .mu.m diameter dots on a pitch of 1 millimeter
mm), but can also be applied to form a pattern of spaced-apart
lines. Additionally, a metallic back contact layer 870 (for
example, Al) is deposited over patterned overlayer stack 830' and
lbsf regions 850 to make the completed solar cell structure of FIG.
9I.
Exemplary fabrication conditions and materials characteristics of
the laser doped regions of FIG. 1 through FIG. 5 will now be
described in additional detail for two specific examples: (i) a
rastered dot irradiation to form blanket laser bsf regions (such as
might be performed on the structure of FIG. 6A), and (ii) a step
and repeat dot irradiation to form localized laser bsf regions and
overlayer patterning (such as might be performed on the structure
of FIG. 2A). Both examples can utilize the same dopant-containing
amorphous silicon layer stack 102/104 that includes i-aSiH layer
102 and B-doped p-aSiH layer 104, deposited at 250 C by PECVD using
13.56 MHz plasma excitation. The i-aSiH layer can be deposited by
PECVD on an HF-last cleaned Si surface from a 1:10 mixture of
SiH.sub.4 and H.sub.2 at a pressure of 8 Torr and the B-doped
p-aSiH layer deposited on the i-aSiH from a 1:3.4:11 mixture of
SiH.sub.4, B.sub.2H.sub.6, and H.sub.2 at a pressure of 4 Torr. The
thickness of the i-aSiH layer can be, in this example, fixed at 10
nm whereas the thickness of the B-doped p-aSiH layer can be in a
thickness range of 10 to 100 nm, with 20-50 nm being
preferable.
For example (ii), the overlayer stacks can include one or more
layers of PECVD SiO.sub.2 and/or SiN, deposited at 250-400 C from
mixtures of SiH.sub.4/N.sub.2O or SiH.sub.4/N.sub.2, typically to a
combined thickness in the range 80 to 110 nm. These layer stacks
provide excellent passivation, with minority carrier lifetimes of
2-5 milliseconds (ms) measured (by microwave photoconductance) for
p-type Czochralski-grown (CZ) Si wafers (surface orientation 100,
resistivity 18 ohm-cm, thickness 720 .mu.m) coated on both
sides.
Additionally, patterned irradiation can be primarily provided by a
diode pumped Q-switched laser providing irradiation to a
.about.40-50 .mu.m diameter spot in 20-30 ns pulses at a repetition
rate of 50-60 kHz, average powers of 4-13 W, and wavelengths of 532
or 1064 nm. By way of example, the laser spot position can be fixed
and the sample mounted on a translation stage. For rastered samples
(used to produce blanket bsfs), a sample stage can be scanned at 10
cm/sec in a back-and-forth pattern under the laser spot to draw
parallel lines spaced apart by 40 .mu.m, a procedure which provided
lines of overlapping spots with a center-to-center spacing equal to
the scan rate divided by the laser repetition rate (for example, 2
.mu.m center-to-center spacing for a scan rate of 10 cm/sec and a
repetition rate of 50 kHz). Samples to be patterned with
spaced-apart dots (for localized bsfs) can be exposed in a
step-and-repeat mode in which the sample is stationary while being
exposed to a selected number of pulses (typically 5 to 20) before
being moved for irradiation at the next spot location (typically 1
mm away). Example conditions can include wavelength 532 nm, 5-6 W
average power, repetition rate 60 kHz, and scan rate 10 cm/sec for
the scanned samples, and N=5 for the step-and-repeat samples.
Sheet resistance (Rs) measurements (4-point probe, after correction
for substrate conductivity) for these rastered conditions on the CZ
Si wafers described above can show, for example, approximately 9
ohm/sq for a 50 nm B-doped p-aSiH film, with the expected inverse
Rs scaling with p-aSiH film thickness (for example, 25 ohm/sq for a
p-aSiH film thickness of 20 nm). In one example, the Rs values were
unaffected by the presence of the SiN and/or SiO.sub.2 overlayers
used. Secondary ion mass spectrometry (SIMS) analysis indicated the
B concentration to be 6e21/cm.sup.3 in the as-deposited p-aSiH
films. Also, after irradiation, the dopant profile was
approximately box-like, with a depth of 0.5 .mu.m and an average
concentration of around 5e20/cm.sup.3.
While some above-described embodiments of the invention have
focused on photovoltaic (PV) applications such as blanket laser BSF
and localized laser BSF (dot pattern, lines, grids, etc.), blanket
laser emitter and selective laser emitter embodiments of the
invention (on either the front or back surface of a solar cell) are
possible as well. By way of example, a selective laser emitter
embodiment of the invention can be implemented on p-type substrates
by forming a blanket crystalline n-type layer by POCl.sub.3
diffusion, depositing a passivating n-type dopant-containing
amorphous silicon layer stack and ARC overlayer on the blanket
crystalline n-type layer, and patterning the amorphous silicon
layer stack and ARC overlayer to form a heavily-doped crystalline
selective emitter in a finger/grid pattern under openings in the
ARC layer. Analogous embodiments may be implemented in n-type
substrates with the substitution of layers with opposite doping
types.
While the techniques described and illustrated above have been
applied to standard homojunction crystalline Si solar cells (that
is, non-HIT cells), embodiments of the invention can also be
implemented in HIT cells as well. As described, for example, by M.
Taguchi et al. in "HIT.TM. cells--high efficiency crystalline Si
cells with novel structure" [Prog. Photovolt: Res. Appl. 8: 503-513
(2000)], HIT cells typically include an i-aSiH/doped-aSiH stack of
one doping type on one side of a semiconductor substrate to
function as a blanket emitter and an i-aSiH/doped-aSiH stack of
opposite doping type on the opposite side of the substrate to
function as a blanket back surface field. Both dopant stacks are
coated with a transparent conductor and metallic finger/bus grid.
As noted above, a challenge with HIT cells is the narrow process
window for providing i-aSiH/doped-aSiH stacks with aSiH layers
thick enough to provide good passivation yet thin enough to provide
sufficient tunneling current to the back surface metallurgy.
In an embodiment of the invention, localized crystalline regions of
high dopant concentration (and conductivity) may be created from
the i-aSiH/doped-aSiH layer stacks already in the cell structure.
These localized regions provide a parallel, high-conductivity path
to the back surface metallurgy (typically, as noted above, a
blanket layer of transparent conductor on which is disposed a
metallic finger/grid pattern). In particular, such localized
regions may be incorporated into the emitter side of a conventional
HIT cell, the bsf side of a conventional HIT cell, or both the
emitter and bsf sides of a conventional HIT cell. These localized
regions may also be incorporated into the HIT side of hybrid HIT
cells in which there is a HIT structure on one side of the cell and
a conventional structure (for example the cell front structure of
FIG. 5A through FIG. 5G) on the other side of the cell.
FIG. 10A through FIG. 10G include a mix of cross-section and
plan-view diagrams illustrating an example process schematic for
the particular case of forming a HIT-type solar cell incorporating
the laser-doped crystalline regions of the invention as part of a
bsf. Specifically, FIG. 10A depicts in cross-section view a
starting p-type Si substrate 900. FIG. 10B depicts the structure of
FIG. 10A after deposition of an i-aSiH/n-aSiH amorphous silicon
stack 902/904 on an upper surface of p-type substrate 900 and an
i-aSiH/p-aSiH stack 906/908 on a lower surface of p-type substrate
900, where the thicknesses of i-aSiH and doped aSiH layers are
typically in the range 2-20 nm. FIG. 10C depicts the structure of
FIG. 10B after laser patterned laser irradiation to form localized
heavily doped crystalline p-type regions 912 on a back surface of
substrate 900 (which may be a pattern of dots or grid lines, as
indicated in the bottom plan view FIG. 10F and FIG. 10G).
FIG. 10D depicts the structure of FIG. 10C after application of
back and front transparent conductor layers 974 and 974'. The
completed cell structure, formed by depositing back and front
metallic grids 976 and 976' on the structure of FIG. 10D, is shown
in FIG. 10E. It should be readily apparent to those skilled in the
art how the process flows of FIG. 9A through FIG. 9I and FIG. 10A
through FIG. 10E may be modified for fabricating any of the
alternative HIT cell embodiments mentioned herein.
FIG. 11 is a flow diagram illustrating techniques for forming at
least one doped, conductive crystalline region on a surface of a
crystalline semiconductor layer (for example, a localized laser
back surface field region of a solar cell), according to an
embodiment of the present invention. Step 1102 includes forming a
dopant-containing amorphous silicon layer stack (for example, a
lower layer of undoped amorphous silicon providing a passivating
function and an upper layer of doped amorphous silicon providing a
dopant source function) on at least one portion of a surface of a
crystalline semiconductor layer. The amorphous silicon layer stack,
as detailed herein, includes a purely amorphous phase as well as an
amorphous phase with embedded nanocrystalline and/or
microcrystalline regions.
The forming step can include, for example, forming an
i-aSiH(bottom)/p-aSiH(top) bi-layer on the portion of the surface
of the crystalline base semiconductor layer, where the undoped
(intrinsic) i-aSiH passivating layer may be 10 nm thick and the
p-type dopant source layer p-aSiH may be a B-doped aSiH layer that
is 10 to 50 nm thick. Also, in another embodiment of the invention
the B-doped aSiH layer can have a thickness of approximately 2-20
nanometers i-aSiH+10-50 aSiHB. Also, at least one embodiment of the
invention can include forming a transparent layer or layer stack on
the aSiH(bottom)/doped-aSiH(top) layer, as well as forming a
conductive contact layer on the transparent layer stack. Further,
at least one embodiment of the invention includes irradiating at
least one selected area of the dopant-containing amorphous silicon
layer stack to form at least one localized crystalline region of
high doping concentration over which the transparent layer stack
has been removed.
Additionally, the dopant-containing amorphous silicon layer stack
can include a layer of undoped amorphous silicon providing a
passivating function and a layer of doped amorphous silicon
providing a dopant source function. The layer of undoped amorphous
silicon and/or the layer of doped amorphous silicon can include
a-SiH, a-Si(Ge)H, a-Ge(Si)H, and/or a-GeH, wherein H content varies
from approximately 5% to approximately 50% atomic percent. Also,
the layer of undoped amorphous silicon and/or the layer of doped
amorphous silicon can additionally include carbon (C) and/or one or
more dopants selected from a group including boron (B), phosphorous
(P), arsenic (As), antimony (Sb), nitrogen (N), gallium (Ga),
indium (In), and aluminum (Al).
Step 1104 includes irradiating at least one selected area of the
dopant-containing amorphous silicon layer stack, wherein the
dopant-containing amorphous silicon layer stack interacts with an
upper portion of the underlying semiconductor layer to form a
doped, conductive crystalline region in the at least one irradiated
area. The irradiating step can include providing laser radiation to
locally heat at least one selected area of the layer stack and one
or more underlying base semiconductor layer regions. Additionally,
the irradiating step can include irradiating at least one selected
area of the silicon layer stack and one or more underlying base
semiconductor layer regions to form at least one localized region
of high doping concentration as well as a self-aligned opening in
the overlayer stack above said localized crystalline region.
Additionally, the overlayer stack can include at least one metallic
layer that is reflowed over one or more edges of said self-aligned
opening to make contact with said localized crystalline region.
Further, at least one embodiment of the invention includes forming
a conductive contact layer over the overlayer stack and exposed
localized crystalline regions remaining after irradiation.
The irradiating step can also include irradiating a pattern
selected from a group including a blanket pattern, a grid pattern,
a finger and/or bus pattern, and a spaced-apart dots pattern. The
irradiating step is preferably provided by one or more pulses of a
pulsed laser (pulse length <100 ns) so as to reduce thermal
diffusion and bulk substrate heating.
The techniques depicted in FIG. 11 can also include forming a
blanket transparent conductor layer over the dopant-containing
amorphous silicon layer stack and localized crystalline regions
remaining after irradiation, and forming a metallic conductor layer
on the transparent conductor layer, wherein the metallic conductor
has a grid pattern or a blanket pattern.
As detailed herein, the dopant-containing amorphous silicon layer
stack remains intact in non-irradiated areas. The techniques
depicted in FIG. 11 can additionally include selecting a p-type Si
substrate. Further, at least one embodiment of the invention
includes forming an overlayer above the dopant-containing amorphous
silicon layer stack formed in step 1102. The overlayer can include
a single or multilayer ARC or dielectric coating, a back reflector,
a diffusion barrier, and/or a transparent conductor. After the
irradiation of step 1104, the overlayer would include one or more
openings self-aligned to the at least one doped, conductive
crystalline region formed via said irradiating step.
Additionally, as described herein, at least one embodiment of the
invention includes a structure that includes a crystalline
semiconductor having at least one surface, a doped crystalline
region disposed in at least one selected area of the semiconductor
surface, and a dopant-containing amorphous silicon layer stack
containing a same dopant as present in the doped crystalline region
on at least a portion of the semiconductor surface outside the
selected area, wherein the dopant-containing amorphous silicon
layer stack passivates the portion of the semiconductor surface on
which it is disposed. The structure can additionally include an
overlayer formed above the silicon layer stack, wherein the
overlayer includes a single or multilayer antireflection coating
(ARC), a back reflector, a diffusion barrier, and/or a transparent
conductor.
As detailed herein, such a structure can include a blanket layer of
a metallic conductor disposed on the dopant-containing amorphous
silicon layer stack and the doped crystalline region. Accordingly,
such a structure can be one face of a solar cell structure.
Additionally, at least one embodiment of the invention can also
include a blanket layer of a transparent conductor disposed on the
dopant-containing amorphous silicon layer stack and the doped
crystalline region, and a metallic conductor layer disposed on the
blanket layer of transparent conductor wherein the metallic
conductor layer has a grid pattern or a blanket pattern. This
particular structure can also be one face of a solar cell
structure.
Further, a structure of at least one embodiment of the invention
can include a patterned overlayer stack disposed on the
dopant-containing amorphous silicon layer stack, wherein the
patterned overlayer stack is patterned with at least one opening
over the doped crystalline region, and a metallic conductor layer
disposed over the patterned overlayer stack and doped crystalline
region. The overlayer stack can include at least one of a single or
multilayer dielectric coating, a back reflector, a diffusion
barrier, a transparent conductor, and a metallic conductor. Such as
structure can also be one face of a solar cell structure.
In such a structure, the dopant-containing amorphous silicon layer
stack can include a layer of undoped amorphous silicon providing a
passivating function and a layer of doped amorphous silicon
providing a dopant source function. The layer of undoped amorphous
silicon and/or the layer of doped amorphous silicon can include, as
noted herein, one of a-SiH, a-Si(Ge)H, a-Ge(Si)H, and a-GeH,
wherein H content varies from approximately 5 to approximately 50
atomic percent. Further, the layer of undoped amorphous silicon
and/or the layer of doped amorphous silicon in such a structure can
also include C and/or one or more dopants selected from a group
including B, P, As, Sb, N, Ga, In, and Al.
Also, in at least one embodiment of the invention, the structure
can include a conductive overlayer formed over the structure, and
the structure can be one face of a solar cell structure.
It is also to be appreciated that all or portions of at least one
embodiment of the present invention may be implemented in a wide
variety of PV and non-PV devices, PV device geometries (including
interdigitated back contact geometries, bifacial cell geometries,
front surface field/emitter-on-the-back geometries, etc.), and PV
fabrication schemes.
The resulting PV and non-PV devices may be distributed by the
fabricator as single cells or devices in raw form, as single cells
or devices with packaging, or as single cells or devices in a
multichip package that may include PV devices and components
functionalities other than PV.
FIG. 12 is a flow diagram illustrating techniques for forming a
doped, conductive crystalline region on a surface of a crystalline
semiconductor layer (for example, a laser back surface field region
of a solar cell), according to an embodiment of the present
invention. Step 1202 includes forming a dopant-containing amorphous
silicon layer stack on at least one portion of a surface of a
crystalline semiconductor layer. The techniques depicted in FIG. 12
can additionally include selecting a p-type Si substrate.
Step 1204 includes irradiating the dopant-containing amorphous
silicon layer stack, wherein the dopant-containing amorphous
silicon layer stack interacts with the underlying crystalline
semiconductor layer to form a blanket doped, conductive crystalline
region. The irradiating step is preferably provided by one or more
pulses of a pulsed laser (pulse length <100 ns) so as to heat
the dopant-containing amorphous silicon layer stack with a minimum
of bulk substrate heating.
Although illustrative embodiments of the present invention have
been described herein with reference to the accompanying drawings,
it is to be understood that the invention is not limited to those
precise embodiments, and that various other changes and
modifications may be made by one skilled in the art without
departing from the scope or spirit of the invention. The
terminology used herein was chosen to best explain the principles
of the embodiments, the practical application or technical
improvement over technologies found in the marketplace, or to
enable others of ordinary skill in the art to understand the
embodiments disclosed herein.
* * * * *